The present invention relates to a method of attenuating unwanted electric field radiation which is radiated from the front screen of a cathode ray tube display and a cathode ray tube display having reduced unwanted electric field radiation emitted from a front screen thereof.
Unwanted electric field radiation from cathode ray tube displays is a subject of concern. For example, low frequency electric field radiation (5 Hz to 400 kHz) is banned by various regulations including TCO and MPR (SSI) of Sweden. It is said that electromagnetic waves derived from the electric field may be hazardous to human and animal bodies as well as cause faults or errors in peripheral electronic devices.
One source of electric field radiation generated by a cathode ray tube display is a high-voltage ripple which has a horizontal deflection period and is generated in a high voltage circuit for supplying a high voltage to an anode electrode of the cathode ray tube. Another source of electric field radiation generated by a cathode ray tube display is electrostatic induction by a pulse voltage which is output from a deflection circuit and appears at a horizontal deflection coil and has a horizontal deflection period. The present invention eliminates the radiation from the electrostatic induction by the pulse voltage generated in the horizontal deflection coil.
A conventional method of attenuating the radiation resulting from high-voltage ripple which has a horizontal deflection period and is generated in a high voltage circuit is disclosed by the applicant in Japanese Patent Application 104893/1994 (Japanese Patent Laid-open Publication 288831/1995). Referring to FIG. 4, this conventional method of attenuating electric field radiation provides a winding 10 as a means for producing a negative pulse which has a horizontal deflection period, at a secondary side M2 of a flyback transformer 13 which is a major component of a high voltage circuit 3. The high voltage circuit 3 is arranged such that the negative pulse produced by the winding 10 of the negative pulse generating means is superimposed on a high voltage HV through a smoothing capacitor 11 connected to the output of the flyback transformer 13. Thus, the high-voltage ripple which has a horizontal deflection period is cancelled. This arrangement also allows unwanted electric field radiation generated by a pulse voltage which has a horizontal deflection period and is generated from a horizontal deflection coil, which is a member of a deflection yoke and so on, to be attenuated. As a result, electric field radiation from the cathode ray tube display is attenuated.
When the conventional unwanted electric field attenuating method is applied to a cathode ray tube display having a deflection circuit and a high voltage circuit separately provided, the horizontal deflection period pulses are generated respectively. The horizontal deflection period pulse voltages of the two separate circuits are determined independently for optimum setting and are different from each other in the pulse width, amplitude, and phase.
More specifically, even if the two, deflection and high voltage, circuits are driven concurrently by a single drive pulse signal, their horizontal deflection period pulse voltages generated respectively in each circuit do not have the same pulse width, amplitude, and phase due to difference between their components or operating conditions. For example, when the high voltage circuit employs as an output element an FET which has no storage time, its horizontal deflection period pulse voltage will resultantly be out of phase with the horizontal deflection period pulse voltage of the deflection circuit.
In the conventional unwanted electric field attenuating method, while radiation from the high voltage ripple in the horizontal deflection period is rationally attenuated, another unwanted radiation caused by the electrostatic induction by the horizontal deflection period pulse voltage of the deflection circuit, including a deflection yoke, is negligibly reduced.
Referring to FIG. 3, the electric field caused by the electrostatic induction of a pulse voltage which has a horizontal deflection period and generated from the deflection circuit including the deflection yoke and radiated from the front screen of the cathode ray tube display is explained. As shown, a deflection yoke 5 is mounted on a cathode ray tube 6 and, in a normal drive mode, is loaded with a horizontal deflection period pulse voltage which has a waveform denoted by A. As the deflection yoke 5 is directly mounted to the cathode ray tube 6, electrostatic induction occurs at the internal conductive coating 2 of the cathode ray tube 6 by electrostatic induction coupling with the pulse voltage in the deflection yoke 5. Accordingly, a pulse voltage, with a wave form denoted by A1 in FIG. 2, is radiated as the unwanted electric field from the front screen of the cathode ray tube 6.
It is an object of the invention to eliminate the foregoing problem by providing a method for attenuating electric field radiation caused by electrostatic induction on an internal conductive coating of a cathode ray tube by a pulse voltage which is produced by a deflection yoke in a deflection circuit and has a horizontal deflection period, and to provide a cathode ray tube display having reduced electric field radiation emitted from front screen thereof.
According to a feature of the present invention a method is provided wherein, a pulse voltage B1, which is reverse polarity to and synchronized with a pulse voltage output A1 of a deflection circuit in a cathode ray tube display, is produced and applied to an internal conductive coating of a cathode ray tube through an electrostatic capacitance formed between the internal conductive coating and an external conductive coating of the cathode ray tube by impressing a pulse voltage B, which is reverse in polarity to and synchronized with a pulse voltage output A of the deflection circuit, on the external conductive coating so as to cancel the ripple voltage A1 which has the horizontal deflection period and is generated from the deflection yoke. As a result, unwanted electric field radiation from a front screen of the cathode ray tube is attenuated.
The pulse voltage B which is reverse in polarity to and synchronized with the pulse voltage output A of the deflection circuit is optionally impressed on a conductive film placed directly over the external conductive coating.
According to another feature of the present invention, a cathode ray tube display includes a cathode ray tube having an internal conductive coating, an external conductive coating and a deflection yoke mounted on the cathode ray tube, and a reverse pulse voltage impressing means having a cancellation transformer with a secondary winding coupled to the external conductive coating via a capacitor. The reverse pulse voltage impressing means impresses on the internal conductive coating, through an electrostatic capacitance existing between the internal conductive coating and the external conductive coating, a pulse voltage B1 which is reverse in polarity to a pulse voltage A1 applied to the internal conductive coating through an electrostatic capacitance formed between the deflection yoke and the internal conductive coating.
It is preferable that the reverse pulse voltage impressing means includes a variable resistor for controlling the amplitude of the pulse voltage B1.
As the pulse voltage B, which is reverse in polarity to and synchronized with a pulse voltage output A of the deflection circuit in a cathode ray tube display, is impressed on the external conductive coating, electrostatic induction produces a pulse voltage B1 which is reverse in polarity to and synchronized with a pulse voltage output A1 of the deflection circuit at the internal conductive coating through the electrostatic capacitance formed between the internal conductive coating and the external conductive coating. Accordingly, unwanted electric field radiation from the front screen of the cathode ray tube display is attenuated.